218 Cf. also the curious phenomenon in a dividing egg described as ãspinningã by Mrs G. F. Andrews, J. of Morph. XII, pp. 367ã389, 1897.
219 Whitman, J. of Morph. II, p. 40, 1889.
220 ãSouvent il nãy a quãune sûˋparation physique entre le cytoplasme et le suc nuclûˋaire, comme entre deux liquides immiscibles, etc.;ã Alexeieff, Sur la mitose dite ãprimitive,ã Arch. f. Protistenk. XXIX, p. 357, 1913.
221 The appearance of ãvacuolationã is a result of endosmosis or the diffusion of a less dense fluid into the denser plasma of the cell. Caeteris paribus, it is less apparent in marine organisms than in those of freshwater, and in many or most marine Ciliates and even Rhizopods a contractile vacuole has not been observed (Bû¥tschli, in Bronnãs Protozoa, p. 1414); it is also absent, and probably for the same reason, in parasitic Protozoa, such as the Gregarines and the Entamoebae. Rossbach shewed that the contractile vacuole of ordinary freshwater Ciliates was very greatly diminished in a 5 per cent. solution of NaCl, and all but disappeared in a 1 per cent. solution of sugar (Arb. z. z. Inst. Wû¥rzburg, 1872, cf. Massart, Arch. de Biol. LX, p. 515, 1889). Actinophrys sol, when gradually acclimatised to sea-water, loses its vacuoles, and vice versa (Gruber, Biol. Centralbl. IX, p. 22, 1889); and the same is true of Amoeba (Zuelzer, Arch. f. Entw. Mech. 1910, p. 632). The gradual enlargement of the contractile vacuole is precisely analogous to the change of size of a bubble until the gases on either side of the film are equally diffused, as described long ago by Draper (Phil. Mag. (n. s.), XI, p. 559, 1837).
Rhumbler has shewn that contractile or pulsating vacuoles may be well imitated in chloroform-drops, suspended in water in which various substances are dissolved (Arch. f. Entw. Mech. VII, 1898, p. 103). The pressure within the contractile vacuole, always greater than without, diminishes with its size, being inversely proportional to its radius; and when it lies near the surface of the cell, as in a Heliozoon, it bursts as soon as it reaches a thinness which its viscosity or molecular cohesion no longer permits it to maintain.